Polymerizable liquid crystal materials are known in prior art for the preparation of anisotropic polymer films with uniform orientation. These films are usually prepared by coating a thin layer of a polymerizable liquid crystal mixture onto a substrate, aligning the mixture into uniform orientation and polymerizing the mixture. The orientation of the film can be planar, i.e. where the liquid crystal molecules are oriented substantially parallel to the layer, homeotropic (rectangular to the layer) or tilted.
For specific applications it is required to induce planar alignment in the liquid crystal layer. The alignment is then frozen in by polymerizing the liquid crystal mixture in situ. For example, oriented films or layers of polymerized nematic liquid crystal material with planar alignment are useful as A-plate compensators or polarizers. Another important application is oriented films or layers of polymerized cholesteric liquid crystal material having twisted molecular structure. If the cholesteric material has planar alignment, these films show selective reflection of light where the reflection color is dependent on the viewing angle. They can be used for example as circular polarizers, color filters or for the preparation of effect pigments for decorative or security applications.
Planar alignment can be achieved for example by treatment of the substrate onto which the liquid crystal material is coated, such as rubbing or application of alignment layers, or by applying shear forces to the liquid crystal material, for example during or after coating.
It is also known in prior art that planar alignment of a liquid crystal material on a single substrate, with one surface of the liquid crystal material being open to the air, can be achieved or enhanced by addition of a surface active compound to the liquid crystal material.
In contrast homeotropic alignment in layers of liquid crystal material is not enhanced by surface active agents. Thus, these agents are generally not employed in LC mixtures for homeotropic aligned layers. Adding a surface active agent for planar alignment into a homeotropic layer can be even detrimental to the uniform alignment.
Molecules with a low surface energy readily accumulate at the LC/air interface promoting the planar orientation effect as reported for example in U.S. Pat. No. 5,344,956. In practice, once polymerized the LC films may be part of a composite LC cell, in which other layers need to be added to the polymer LC film, such as, but not restricted to, other liquid crystal, adhesive or barrier layers.
In prior art the use of Fluorad FC171® (a non-reactive, fluorocarbon surfactant from 3M, St. Paul, Minn.) and the fluorocarbon acrylates FX13® (a monoacrylate from 3M), Zonyl® 8857A (DuPont) and PolyFox® PF-3320 (Omnova Solutions Inc.) have been disclosed in EP 1256617 A1 and US 2004/0202799 A1.

In EP1256617 A1a reactive surfactant has been used as an additive to modify the reactive mesogen (RM) mixture. This concept has also, more recently, been reported in US 2004/0202799. In all cases, the reactive surfactant was chosen so that it would react with the RMs in the mixture and hence be locked in place. As such, the surfactant is not free to migrate from the RM layer. This is desired when manufacturing stacks of films (as highlighted in EP 1256617). Due to the low surface tension of surfactant materials any non-polymerized surfactants may readily migrate into a subsequent layer. This can be prevented by the use of a mono-reactive surfactant, e.g. FX13®.
For example, WO 99/45082 describes an optical retardation film that is obtained from a layer of polymerizable liquid crystal material with planar alignment comprising one or more fluorocarbon surfactants. U.S. Pat. No. 5,995,184 reports a method of making a phase retardation plate from a layer of polymerizable liquid crystal material with planar alignment, where a surface active material, like for example a polyacrylate, polysilicone or organosilane, is added to the liquid crystal material to reduce the tilt angle at the liquid crystal/air interface of the liquid crystal layer.
US 2002/0111518 A1 discloses fluorinated multifunctional acrylates for the use in coating compositions. There is no reference made to RM films. A synthesis of the compounds is disclosed.
Problem
Low diacrylate content RM films are often used where good adhesion of the RM film to the substrate is important. However, the optical retardation of low diacrylate content RM films drops significantly under accelerated conditions, typically at an elevated temperature of 90° C. in a dry atmosphere. This effect is much worse when the film is tested in contact with PSA (pressure sensitive adhesive), rather than just the open film itself. One aim of the present invention is therefore to improve the stability of low-diacrylate films.
Examination of FIG. 1 reveals that most of the drop in retardation of the film occurs at the beginning of the experiment. It is possible that some additives in the PSA are leaching into the RM film and either disturbing the alignment near to the surface of the film or even swelling the bulk film. One common method to prevent any migration of additives into the RM material (RMM) film is to coat the polymerized RMM layer with a hard coat (sometimes called a topcoat). Such layers usually contain small molecules which are cured after coating to build a rigid polymer network. Unfortunately, the use of a hard coat layer requires extra processing steps (e.g. GB 2398077), which are costly and therefore undesirable.
It was therefore an aim of the present invention to provide a polymerizable liquid crystal material for the preparation of polymer films that does not have the drawbacks described above. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
Definition of Terms
Polymerizable compounds with one polymerizable group are also referred to as “monoreactive” compounds, compounds with two polymerizable groups as “direactive” compounds, and compounds with more than two polymerizable groups as “multireactive” compounds. Compounds without a polymerizable group are also referred to as “non-reactive” compounds.
The term “reactive mesogen” (RM) means a polymerizable mesogenic or liquid crystal compound. Materials comprising more or less of reactive mesogens (RMs) are also addressed herein as RM material (RMM).
The term ‘film’ as used in this application includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.
The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal or mesogenic compound’ should denote materials or compounds comprising to a certain percentage one or more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e. groups with the ability to induce liquid crystal phase behavior. Liquid crystal compounds with rod-shaped or board-shaped groups are also known in the art as ‘calamitic’ liquid crystals. Liquid crystal compounds with a disk-shaped group are also known in the art as ‘discotic’ liquid crystals. The compounds or materials comprising mesogenic groups do not necessarily have to exhibit a liquid crystal phase themselves. It is also possible that they show liquid crystal phase behavior only in mixtures with other compounds, or when the mesogenic compounds or materials, or the mixtures thereof, are polymerized.
For the sake of simplicity, the term ‘liquid crystal material’ or ‘LC material’ is used hereinafter for both liquid crystal materials and mesogenic materials, and the term ‘mesogen’ is used for the mesogenic groups of the material.
The director means the preferred orientation direction of the long molecular axes (in case of calamitic compounds) or short molecular axis (in case of discotic compounds) of the mesogens in a liquid crystal material.
The term ‘planar structure’ or ‘planar orientation’ refers to a layer or film of liquid crystal material wherein the director is substantially parallel to the plane of the film or layer.
The term “homeotropic structure” or “homeotropic orientation” refers to a film wherein the optical axis is substantially perpendicular to the film plane.
The term “cholesteric structure” or “helically twisted structure” refers to a film comprising LC molecules wherein the director is parallel to the film plane and is helically twisted around an axis perpendicular to the film plane.
For sake of simplicity, an optical film with twisted, planar or homeotropic orientation or structure is hereinafter also referred to as “twisted film”, “planar film” or “homeotropic film” respectively.
The term ‘A plate’ refers to an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis oriented parallel to the plane of the layer.
The term ‘C plate’ refers to an optical retarder utilizing a layer of uniaxially birefringent material with its extraordinary axis perpendicular to the plane of the layer.
The terms ‘hard coat’ or ‘topcoat’ refer to an additional layer providing extra physical or chemical properties to another layer underneath. The hard coat or topcoat can be an integral part of the layer underneath in terms of the manufacturing process. According to the invention the hard coat or topcoat and the underlying layer are preferably made from a single composition. Accordingly there may be only a gradual change in the properties of the formed stack of layers. In this case there may be no sharp line between the hard coat and the layer underneath. This is not detrimental to the bulk characteristics of the underneath layer nor to the surface characteristics of the hard coat.